Resistivity sounding is a well known geophysical method used in geological applications on land as well as on water. Although the method is generally well accepted for land applications it tends to be less successful in applications on water requiring for instance accurate determination of depths and thicknesses of sea and river sediments and other geological structures.
Many existing electrode configurations have been derived from the “Normal Schlumberger” electrode configuration as originally developed for resistivity sounding on land in which an electrical current is injected into the subsurface by means of two current electrodes. The voltage gradient associated with the electrical field of this current is measured between two voltage electrodes placed in between tie current electrodes. Based on the measured values of current and voltage, the average resistivity of the subsurface is calculated for a subsurface volume down to a certain penetration depth. The penetration depth depends on the distance between the current electrodes and the center point between the voltage electrodes. Larger electrode distances arc associated with increasing penetration depths.
In order to obtain information from progressively deeper geological structures the current electrodes are consecutively shifted while the voltage electrodes remain fixed and the measurements are repeated with progressively increasing current electrode distances. As such, a field curve is obtained showing resistivity as function of the distance between the current electrodes.
The resistivity of a geological structure depends on e.g. its porosity, water saturation and water resistivity. For instance, gravel has usually a lower porosity than sand and thus a higher resistivity, clay generally has very high porosities and thus shows very low resistivities, and solid rock has a low porosity and thus shows very high resistivities. Accordingly, every geological structure has its own specific resistivity.
Resistivity surveys on water are carried out using either a floating cable or a bottom towed cable carrying electrodes in a well defined configuration. For these resistivity surveys on water several electrode configurations are currently used comprising a pair of current electrodes and multiple voltage electrodes.
A commonly used configuration is the dipole-dipole electrode configuration using a pair of current electrodes and several pairs of voltage electrodes located outside the current electrodes. An example of such an electrode array is described in U.S. Pat. No. 4,298,840 (Bischoff et al., Nov. 3, 1981). This patent also discloses a manner to construct the electrodes. The dipole-dipole configuration is known to be very sensitive to lateral subsurface variations and is therefore less appropriate for applications involving inversion of apparent resistivity data into true resistivity depth sections.
One of the currently most commonly used configurations is the “Asymmetric Inverse Schlumberger” configuration where the voltage electrodes are located outside the current electrode pair.
During the sounding, an electrical current is injected between the current electrodes and simultaneously the electric field is measured between one of the voltage electrodes and one of the other voltage electrodes located at the opposite side of the pair of current electrodes. The penetration depth is gradually increased from the voltage electrode closest to the current electrodes towards the voltage electrode most remote from the current electrodes.
Compared to the “Normal Schlumberger” configuration the “Asymmetric Inverse Schlumberger” configuration offers the advantage that resistivity soundings can be carried out much faster because the current injection electrodes remain the same throughout the sounding.
However, the “Asymmetric Inverse Schlumberger” configuration has also a number of disadvantages. The minimum distance that a voltage electrode pair can approach the distorted electrical field in the immediate surroundings of the current electrodes is determined by the ratio defined by the distance between both current electrodes divided by their average distance from the nearest voltage electrode. If this ratio is chosen too large the electrical gradient can not be considered linear. A very low ratio on the other hand results in very high geometrical factors and thus very poor signal/noise ratios. As a high signal/noise ratio is essential for apparent resistivity data to be successfully inverted into true resistivity depth sections, the “Asymmetric Inverse Schlumberger” configuration, which may be appropriate in qualitative applications involving simple apparent resistivities and/or formation factors, is generally less successful in quantitative resistivity applications involving inversion into sediment depths and thicknesses.